1. Field of the Invention:
The present invention is related to a liquid crystal display device and a method of manufacturing the same, and particularly to a liquid crystal display element having a wide viewing angle characteristic and a method manufacturing it.
2. Description of the Related Art:
Conventionally, a liquid crystal display device using a nematic liquid crystal display element is widely employed in a segmented-figure display element such as a watch, or an electronic calculator. On transparent substrates of such a liquid crystal display device, active elements such as thin film transistors are formed as switching means for selectively driving pixel electrodes which applies a voltage to a liquid crystal, and color filter layers such as red, green and blue are formed so as to display color images. On the basis of the twist angle of a liquid crystal, liquid crystal display systems are classified as follows: (a) an active drive twisted nematic (hereinafter, abbreviated as "TN") liquid crystal display system in which nematic liquid crystal molecules are aligned by being twisted by 90.degree., and (b) a multiplex drive super twisted nematic (hereinafter, abbreviated as "STN") liquid crystal display system in which the twist angle of the nematic liquid crystal molecules is set to be 90.degree. or more to utilize the sharpness of the transmittance-applied voltage characteristic.
In the latter multiplex drive STN liquid crystal display system, there is a peculiar color phenomenon. In the case where a monochrome display is to be conducted, therefore, the multiplex drive STN liquid crystal display system is further classified roughly as follows on the basis of the kind of optical phase difference element: Namely, the multiplex drive STN liquid crystal display system includes (b-1) a two-layered double super twisted nematic liquid crystal display system which uses a display liquid crystal cell and a liquid crystal cell wherein molecules are twist aligned by a twist angle opposite to that of the display liquid crystal cell, and (b-2) a film-addition type liquid crystal display system in which a film having an optical anisotropy is disposed. It is considered that the film-addition type liquid crystal display system of (b-2) is advantageous from the view points of the reduction of weight and cost.
On the other hand, the former active drive TN liquid crystal display system of (a) is classified roughly into (a-1) a normally black system in which a pair of polarizing plates are arranged in such a manner that their polarizing directions are parallel to each other and black is displayed when no voltage is applied to the liquid crystal layer ( the off state ), and ( a-2 ) a normally white system in which a pair of polarizing plates are arranged in such a manner that their polarizing directions are perpendicular to each other and white is displayed during the off state. It is considered that the normally white system of (a-2) is advantageous from the view points of the display contrast, color reproduction, and viewing angle dependence of the display.
In a conventional TN or STN type liquid crystal display device, liquid crystal molecules are aligned in one direction on an alignment film. As typical methods of aligning the directions of liquid crystal molecules in one direction, a rubbing process in which a formed thin film is rubbed with a cloth or the like (Japanese Laid-Open Patent Publication No. 4-57028), a deposition polymerization method in which a raw monomer is deposited on the surface of an electrode to form a polymer film, and an LB film method for forming an LB film (Japanese Laid-Open Patent Publication No. 4-181922) are known.
In a conventional TN type liquid crystal display device or the like, the display characteristic has a viewing angle dependence in which, for example, the contrast of a display image varies depending on the viewing angle. Particularly, it is well known that, when the viewing angle is increased from the normal direction of a display screen in the direction of increasing the display contrast (usually, toward the observer's side) to exceed a certain angle, a phenomenon takes place in which the black and white display is inverted (hereinafter, this phenomenon is referred to as merely "inversion"). These phenomena are caused by the fact that liquid crystal molecules having an anisotropy of refractive index are aligned in one direction with respect to the substrate.
FIGS. 51(a), 51(b), and 51(c) are section views of a conventional TN type liquid crystal display element 201. As shown in FIGS. 51(a), 51(b), and 51(c), for example, the TN type liquid crystal display element is configured so that a liquid crystal layer 204 is sandwiched between a pair of substrates 202 and 203. The liquid crystal molecules 205 of the liquid crystal layer 204 have a configuration in which the initial alignment is twisted by 90.degree. and the liquid crystal molecules are tilted in one direction by a certain angle (pretilt angle). Therefore, an alignment process is conducted so that, when a driving voltage from a power source 206 is applied, the liquid crystal molecules 205 are raised in the same direction as shown in FIG. 51(c). When the liquid crystal molecules 205 are raised during a display of a half tone, therefore, the liquid crystal molecules are tilted in the same direction as shown in FIG. 51(b). When the liquid crystal molecules 205 are viewed in the directions respectively indicated by arrows A and B in the figure which are separated from each other by a relatively wide angle, accordingly, the apparent refractive indices are different from each other. As a result, the contrast of an image as viewed from the direction A is largely different from that as viewed from the direction B, and in an extreme case there arises an abnormal display phenomenon such as the inversion.
In order to improve such viewing angle dependence, an attempt to compensate for the phase difference between an ordinary ray and an extraordinary ray by interposing a phase difference plate (film) in which the direction of one principal index of refraction of the index ellipsoid is parallel to the normal direction of the display screen, between a substrate and a polarizing plate. However, the use of such a phase difference plate exerts only a limited effect on the improvement of the above-mentioned inversion of the black and white display.
Other proposed methods include the pixel split method (Japanese Laid-Open Patent Publications Nos. 57-186735, and 5-203951), the rubbing angle optimization method (Japanese Laid-Open Patent Publication No. 4-221935), and the nonrubbing method (Japanese Patent Publication No. 3-14162). In the pixel split method, a pixel electrode for conducting a display is split into two portions, and the two split pixel electrodes are set so as to have different independent viewing angle characteristics. In the rubbing angle optimization method, two polarizing plates are opposed to each other across a liquid crystal display element, and the angle formed by the polarizing direction of one of the two polarizing plates and the alignment direction of liquid crystal molecules, and the twist angle are set within a predetermined range, thereby preventing the inversion from occurring. In the nonrubbing method, an alignment film which is disposed on one of two opposing substrates through a liquid crystal layer is rubbed, and an alignment film which is disposed on the other substrate is not rubbed, so that liquid crystal molecules are caused to be different in alignment direction by the nonrubbed alignment film.
In a conventional liquid crystal display element, the pixel split method can eliminate the inversion in the vertical direction and viewing angle dependence, but has disadvantages that the contrast is lowered, that black is changed to gray when the viewing angle is increased, and that the display characteristics in the lateral directions are different from those in the vertical directions. A method in which one rubbing process is conducted in each of opposite directions with using a resist so that portions which have undergone two rubbing processes and those which have undergone one rubbing process are produced (Japanese Laid-Open Patent Publication No. 5-203951) has problems that the number of process steps is increased, and that the alignment film is deteriorated when the resist is peeled off and the alignment is disturbed, resulting in lowered display quality by causing disclination lines or the like. The display image shows uneven brightness. The rubbing angle optimization method can eliminate the inversion, but has disadvantages that the viewing angle in the direction opposite to that of the inversion is narrowed, and that the contrast in the normal direction is lowered. The nonrubbing method has a disadvantage that, as shown in FIGS. 5(a) and 5(b), disclination lines DL constituted by the discontinuity of the liquid crystal alignment are conspicuous.
As a display element which does not require a polarizing plate and conducts a display utilizing the light scattering by liquid crystals, a display element using the dynamic scattering (DS) effect or the phase transition (PC) effect is known. Recently, a display element which does not require a polarizing plate nor an alignment process has been proposed. In the proposed display element, using the birefringence of a liquid crystal, the transparency and opaquicity are electrically controlled. According to this display method, basically, the ordinary ray refractive index of liquid crystal molecules is made coincident with the refractive index of the supporting medium. When the liquid crystal molecules are aligned by applying a voltage, the transparent state is displayed, and, when no voltage is applied, the opaque state (light scattering state) due to the disturbance of the alignment of the liquid crystal molecules is displayed.
An example of the proposed methods is disclosed in Japanese National Patent Publication No. 61-502128. In the method, a liquid crystal is mixed with a photocurable or thermosetting resin and the resin is cured so that the liquid crystal phase-separates between a pair of substrates, thereby forming liquid crystal regions in the resin. A liquid crystal element formed by this method is called a polymer dispersed liquid crystal element. As an element in which the viewing angle is improved by using a polarizing plate, Japanese Laid-Open Patent Publications Nos. 4-338923, and 4-212928 disclose display elements in which the above-mentioned polymer dispersed liquid crystal element is sandwiched between crossed polarizing plates. In the display elements, the viewing angle characteristics can largely be improved, but their operating principle is based on depolarization due to scattering and therefore the brightness of the image is about one-half of that of a TN mode liquid crystal display element, thereby producing a problem that the image is not bright enough. Hence, the display elements are of less utility. Japanese Laid-Open Patent Publication No. 5-27242 discloses a method in which the alignment state of a liquid crystal is disturbed by walls and projections of a polymer to produce random liquid crystal domains, thereby improving the viewing angle. In this method, liquid crystal domains are formed at random and the polymer material enters into pixels, and therefore there is a problem that the contrast is lowered to impair the display quality.
As shown in FIGS. 52(a), 52(b), and 52(c), a liquid crystal display element 201a in which the phase separation of a liquid crystal and a polymer material is regularly conducted by utilizing a photomask and which is provided with a polarizing plate includes polymer walls 207 made of the polymer material between substrates 202 and 203. A liquid crystal region 209 surrounded by the polymer walls 207 is formed. Each liquid crystal region 209 includes a plurality of liquid crystal domains 210, and a disclination line 208 is formed at each boundary of the liquid crystal domains 210. In the liquid crystal region 209, when a voltage is applied, liquid crystal molecules 205 are caused by the interaction between the liquid crystal molecules 205 and the polymer walls 207, to rise along the direction of the walls 207. Therefore, the apparent refractive indices of the liquid crystal molecules 205 as viewed in the directions respectively indicated by arrows A and B are substantially equal to each other so that the viewing angle characteristics are largely improved. On the other hand, in order that a radial alignment which is most effective in the improvement of the viewing angle characteristic is realized in a pixel, a wall or pillar made of a polymer must be formed at the center of the pixel so that the area of a liquid crystal area in each pixel is reduced. This produces a problem that the light transmittance in the off-period of a driving voltage is reduced, and hence the brightness of a display image and the contrast are lowered.